The behavior of the three commercially available trileaflet prostheses used for clinical aortic valve replacement are presently under investigation. These bioprostheses are marked with radiopaque markers at various points and implanted in calves. The movement of various parts of the bioprostheses are studied under x-ray over a several week period subsequent to implantation. Finally the bioprostheses are recovered and subjected to light microscopy and ultrastructural studies in order to determine the alterations that have occurred in the tissue. Based on the movement of the radiopaque markers the areas of increased stress in comparison to the natural aortic valve will be determined and the areas of increased stress correlated with the histologic findings. Based on the finding of abnormal stresses an attempt will be made to predict the probable site and cause of fatigue failure or calcification of the prostheses. Comparison of the behavior of bioprostheses with that of the natural valve should suggest possible modifications of the bioprostheses that will improve their lives and performance and also define their limitations.